M. Vural

Microstructural aspects and modeling of failure in naturally occurring porous composites

Abstract Results from an experimental investigation on the compression behavior of balsa wood are presented. Specimens with varying densities, ranging from 55 to 380 kg/m 3 , are loaded in the grain (fiber, cell) direction using a screw-driven material testing system at a strain rate of 10 −3 s −1 . The results indicate that compressive strength of balsa wood increases with increasing density. Post-test scanning electron microscopy is used to identify the failure modes. The failure of low-density specimens is governed by elastic and/or plastic buckling, while kink band formation and end-cap collapse dominate in higher density balsa specimens. Based on the experimental results and observations, several analytical models are proposed to predict the compressive failure strength of balsa wood under uniaxial loading conditions.

Transverse Failure in Thick S2-Glass/ Epoxy Fiber-Reinforced Composites

The transverse mechanical and failure behavior of unidirectional S2-glass/epoxy composites are investigated and reported for a wide range of strain rates from 10-4to 10 4s-1under multiaxial compressive loading conditions. The effect of stress multiaxiality is explored using the confining sleeve technique to impose varying degrees of lateral confinement. The lateral confinement has been found to significantly increase both the maximum attainable stress level and the strain to failure. Scanning electron microscopy (SEM) observations on recovered specimens reveal that the transverse failure occurs within localized shear bands through multiple fiber–matrix interface failure at the micro-scale. Based on theorientation of shear bands a Mohr–Coulomb type failure criterion is suggested to represent the transverse failure of unidirectional S2-glass/epoxy composites.

Plasma-sprayed oxide ceramics on steel substrates

Abstract The behavior of plasma-sprayed oxide ceramic coatings on several metal substrates was investigated under different mechanical and thermal loading conditions. Metallographic evaluations were carried out to determine the structures of the coatings and interfacial regions. Some of the crystallographic transformations were detected by conducting X-ray diffraction analyses on powders and as-sprayed coatings. In alumina-based powders, most of α -Al 2 O 3 (H) always transformed to γ -Al 2 O 3 (C). In the case of magnesium zirconate powders, monoclinic zirconia, which is present in the initial composition, was completely transformed to cubic zirconia. The spraying of lime-stabilized zirconia resulted in the evaporation of CaO powder (CaCO 3 ) because of its relatively lower evaporation point, and part of the monoclinic zirconia was transformed to tetragonal structure after spraying. It was shown by tensile and three-point bending tests that ceramic coatings have a failure mechanism in which microcrack formations prevail and, as the stress level is increased, macrocrack formations start. Thermal shock and flame tests showed that ceramic coatings are resistant to high temperature gradients and they have good thermal barrier properties. However, relatively long-term heat treatment resulted in oxidation problems at the interfacial region in the case of using AISI 1015 carbon steel substrates. For AISI 304 stainless-steel substrates, the failure mechanisms appeared to be the thermal expansion mismatch and partial destabilization of the coating structure.

Influence of Cellular Topology on Dynamic Response of Solid Foams

Current processing techniques enable the manufacture of cellular cores to prescribed cell sizes and densities. Moreover, the rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular topologies tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and topology in cellular structures. The present work reports the initial results of a computational investigation in this direction. The fundamental issues addressed in the present study are (i) generation of stochastic cellular structures by using Voronoi tessellations, (ii) quantitative measure of cellular topology, (iii) uniqueness of mechanical response, (iv) specimen size effect, (v) boundary effect, and (vi) high-strain-rate effects.© 2006 ASME

EBSD characterization of shear band formation in aluminum armor alloys

The shear-compression behavior of four commercial aluminum armor alloys, 2139, 2519, 5083, and 7039, that exhibit enhanced resistance to high-strain-rate deformation, were evaluated using a Split Hopkinson Pressure Bar. Each of the alloys was found to exhibit a characteristic critical equivalent strain beyond which plastic collapse of the material occurred. Microstructural changes were systematically quantified as a function of equivalent strain using electron backscatter diffraction along with the effects of crystallographic orientation, secondary particles, and solid solution strengthening on the accumulation of localized strain within the microstructure. The onset of the plastic collapse was determined to correlate with an equivalent strain where nominally all of the grains within the microstructure exhibited characteristics associated with adiabatic shear band formation. The rapid decline of the flow stress during plastic collapse was found to be enhanced by grain fragmentation and refinement in regions of high stress concentrations. Results from this study suggest that improvements in the performance of these Al armor alloys may potentially be achieved through careful control of their processing, in particular with respect to their texturing and the dispersion of secondary particles in the microstructure.

Multiaxial analysis of dental composite materials

Dental composites are subjected to extreme chemical and mechanical conditions in the oral environment, contributing to the degradation and ultimate failure of the material in vivo. The objective of this study is to validate an alternative method of mechanically loading dental composite materials. Confined compression testing more closely represents the complex loading that dental restorations experience in the oral cavity. Dental composites, a nanofilled and a hybrid microfilled, were prepared as cylindrical specimens, light-cured in ring molds of 6061 aluminum, with the ends polished to ensure parallel surfaces. The samples were subjected to confined compression loading to 3, 6, 9, 12, and 15% axial strain. Upon loading, the ring constrains radial expansion of the specimen, generating confinement stresses. A strain gage placed on the outer wall of the aluminum confining ring records hoop strain. Assuming plane stress conditions, the confining stress (sigma(c)) can be calculated at the sample/ring interface. Following mechanical loading, tomographic data was generated using a high-resolution microtomography system developed at beamline 2-BM of the Advanced Photon Source at Argonne National Laboratory. Extraction of the crack and void surfaces present in the material bulk is numerically represented as crack edge/volume (CE/V), and calculated as a fraction of total specimen volume. Initial results indicate that as the strain level increases the CE/V increases. Analysis of the composite specimens under different mechanical loads suggests that microtomography is a useful tool for three-dimensional evaluation of dental composite fracture surfaces.

Effect of Microstructure in Cellular Solids: Bending vs. Stretch Dominated Topologies

Rapid advance in additive manufacturing techniques promises that, in the near future, the fabrication of functional cellular structures will be achieved with desired cellular microstructures tailored to specific application in mind. In this perspective, it is essential to develop a detailed understanding of the relationship between mechanical response and cellular microstructure. The present study reports on the results of a series of computational experiments that explore the effect cellular topology and microstructural irregularity (or non-periodicity) on overall mechanical response of cellular solids. Compressive response of various 2D topologies such as honeycombs, stochastic Voronoi foams as well as tetragonal and triangular lattice structures have been investigated as functions of quantitative irregularity parameters.

High Strain Rate Testing of Sandwich Core Materials

Mechanical response of a cellular sandwich core material, balsa wood, is investigated over its entire density spectrum from 55 to 380 kg/m3. Specimens were compression loaded along the grain direction in both quasi-static and dynamic strain rates from 10−3 to 103 s−1. Results show that while the initial failure stress is very sensitive to the rate of loading, plateau (crushing) stress remains unaffected by the strain rate. Kinematics of deformation and micro inertial effects are suggested and discussed to explain this different behavior. Specific energy dissipation capacity of balsa wood was measured and determined to be comparable with those of fiber-reinforced polymers. As in quasi-static loading, buckling and kink band formation were identified to be two major failure modes in dynamic loading as well.

Probing the yield surface of lightweight solid foams via biaxial and triaxial experiments

Current work describes and discusses the results from a unique experimental setup called Multi-Axial Testing Apparatus (MATA) that has been developed to probe the yield surface of cellular solids under both biaxial and triaxial stress paths. As a case study, yield surface of Divinycell H100 foam has been probed in its plane of anisotropy by using the MATA. Extensive experimental results that define the yield surface have been reported and also compared to the predictions from an energy-based yield criterion that we have recently proposed for transversely anisotropic solid foams. An excellent match between experimental data points and model predictions is observed and it is concluded that total strain energy density is the driving force of yielding in solid foams.

A new approach to yield prediction in cellular solids

Although there exists several yield criteria proposed in the literature for highly porous solid foams, they are all phenomenological in nature, rely on relatively long list of model parameters that require difficult experimentation not readily available to end user, and none of them can handle the anisotropy observed in commercially available solid foams. Present work offers a novel approach by hypothesizing that the yielding of stochastic foams is governed by the total elastic strain energy density. Resulting analytical framework that is based on transverse anisotropy and energy homogenization leads to a pressure-dependent yield criterion for solid foams. Besides accommodating anisotropy, this energy-based yield criterion renders an advantage by relying only on the elastic properties and uniaxial yield strengths of the material. Finite element analysis is presented for a periodic elongated tetrakaidecahedral cell model with 0.1 relative density to validate the proposed model and the dependence of yield behavior on strain energy density. Proposed yield criterion is also validated by extensive experimental data in a companion paper.